The invention relates to a tachogenerator for detecting the rotational speed of a rotating machine part, in particular that of a vehicle wheel. The tachogenerator has a predetermined measurement resolution and at least one magnetically acting encoder which can rotate with the machine part. It also has magnetically acting areas which are subdivided in a predetermined sequence into angle parts, in particular teeth composed of ferromagnetic material or a magnetic pole ring, and which, during rotation, produces a measurement signal in at least one magnet sensor which is arranged to be stationary relative to the encoder and is separated from said encoder by an air gap. The measurement signal corresponds to the angular pitch of the encoder whose frequency corresponds to the rotational frequency of the machine part.
Present-day tachogenerators, which are used for measuring the wheel rotation speed of motor vehicles, must have a resolution of at least about 40 to 60 pulses per wheel revolution, in particular for use in conjunction with ABS systems, starting aids, and vehicle dynamics and anti-skid control systems. Conventional tachogenerators use an encoder which is attached to the rotating components and is subdivided into a number of hard-magnetic pole pairs corresponding to the resolution of the tachogenerator, or comprises a toothed wheel, which is manufactured from ferromagnetic material and has a corresponding number of teeth. In the latter case, the magnetically sensitive sensor, for example a Hall sensor or a magnetoresistive sensor, is magnetically preloaded by a permanent magnet. The encoder is magnetically coupled to a magnet sensor via an air gap. The continuously changing magnetic field during rotation of the encoder produces a measurement signal, whose frequency corresponds to the rotation frequency of the encoder.
Conventional magnet sensors allow a maximum air gap width of only about 2 mm, since, otherwise, the measurement signal is too weak. This results in considerable manufacturing complexity.
EP 0 806 673 proposes a tachogenerator which uses a GMR sensor (giant magnetoresistive effect). Owing to the increased sensitivity of such a sensor, the tachogenerator operates even with large air gap widths of about 4 mm. The measurement signal emitted by the GMR sensor is twice the frequency of that emitted by a conventional sensor, since the characteristic of the GMR sensor has mirror-image symmetry. Furthermore, smaller air gaps result in the measurement signal having a poor duty ratio. The duty ratio can be stabilized by using a frequency divider to halve the frequency of the measurement signal. The duty ratio of the measurement signal from such GMR sensors is highly dependent on the air gap width, so that, although the tachogenerator operates with large air gap widths, its response varies severely, however, if the air gap does not remain constant. For example, the air gap varies with the load state of the rotating machine part or as its bearing wears.
When such a tachogenerator is fitted to the wheels of a motor vehicle, a large air gap has considerable advantages, since large dynamic forces occur at this point and the structure is correspondingly severely elastically deformed. This results in the geometry and therefore also the air gap of the tachogenerator varying continuously and considerably. The solutions to this problem which have been found so far can be implemented only subject to major costs.
The object of the present invention is to specify a tachogenerator which still operates reliably, even with a comparatively large air gap which is not constant, but which can nevertheless be produced easily and at low cost, and which operates with the necessary resolution.
The object is achieved in that, in order to produce measurement signals which can be evaluated with an air gap which is not constant, the angular pitch of the encoder is coarser than the predetermined measurement resolution requires, and at least two magnet sensors are provided, which are arranged fixed one behind the other in the rotation direction, relative to the encoder, in order to produce at least two mutually phase-shifted measurement signals which correspond to the angular pitch of the encoder, and means are provided for linking the measurement signals emitted by the sensors to form an output signal, with the output signal being at a frequency which is greater than the measurement signals of the magnet sensors, in order to achieve the predetermined measurement resolution.
To this end, a first refinement of the invention provides for the magnet sensors to have Hall elements.
A further refinement of the invention provides for the magnet sensors to have magnetoresistive sensor elements.
By using a plurality of sensors, the required measurement resolution can be achieved even if the angular pitch of the encoder is much smaller than the required resolution of the measurement signal.
When the pitch of the encoder is reduced, this results in a greater magnetic flux density, since the distances between the magnetically acting elements of the encoder are increased. For example, the number of teeth or the number of poles of an encoder toothed wheel composed of ferromagnetic or hard-magnetic material is halved in comparison with the desired resolution. In consequence, the tooth gaps or the poles of the encoder are enlarged, and the amplitude of the measurement signal is in consequence also increased. The individual measurement signals are then linked to one another in such a manner that the angular pitch of the encoder is half the magnitude of the resolution of the tachogenerator, and in such a manner that the frequency of the output signal is twice the frequency of the measurement signals. This can be achieved by simple links which are carried out in a circuit connected to the sensors or in an integrated circuit, or in a microprocessor.
Such a sensor operates reliably with air gap widths of more than 2 mm.
As an alternative solution to the object on which the invention is based, it is proposed that at least two such magnet sensors be provided which are arranged fixed one behind the other in the rotation direction, relative to the encoder, in order to produce at least two mutually phase-shifted measurement signals, which correspond to the angular pitch of the encoder, in the case of which magnet sensors the mathematical sign of their characteristic behaves at least approximately with mirror-image symmetry with respect to the magnetic field strength, that, in order to produce measurement signals which can be evaluated with an air gap which is not constant, the angular pitch of the encoder be coarser than the predetermined measurement resolution requires, and means be provided for linking the measurement signals emitted by the magnet sensors to form an output signal, with the output signal being at a lower frequency than the original measurement signals, in order to achieve a good duty ratio.
Such a characteristic is found, for example, in so-called GMR sensors. As a rule, GMR sensors comprise two or four magnetoresistive sensor elements which are connected on a small chip to form a Wheatstone bridge. The measurement signal from such a sensor is derived from the change in the diagonal resistance of the bridge circuit.
GMR sensors have a mirror-image symmetrical characteristic in comparison with conventional magnetoresistive sensors, so that they emit a measurement signal at twice the frequency of the measurement signal of conventional sensors. However, particularly at high magnetic field strengths, this measurement signal has a poor duty ratio. According to the invention, a stable output signal duty ratio which is independent of the field strength, and thus of the air gap width, is achieved by linking the measurement signals while reducing, for example halving, the frequency of the measurement signals.
The linking of the individual measurement signals according to the invention can be carried out, for example, by means for binary exclusive-OR linking to form the output signal, by means for multiplication to form the output signal, or by means for forming the magnitude of the difference in order to produce the output signal.
One refinement of the invention provides for the measurement signals to be phase-shifted through about 90xc2x0. The signals can thus be easily linked to form an output signal which is at twice the frequency of the measurement signals.
The measurement signals can be linked by the magnet sensors being connected to an exclusive-OR circuit via threshold-value switches, in order to produce the output signal. This arrangement may be used both in conjunction with Hall sensors or magnetoresistive sensors, and with GMR sensors.
In a next refinement of the invention, the measurement signals are linked by the sensors being connected to a multiplication circuit, in order to produce the output signal.
A further refinement of the invention using GMR sensors provides for the sensors to be connected via threshold-value switches to flipflop circuits whose outputs are linked to an exclusive-OR circuit in order to produce the output signal.
Where mirror-image symmetrical GMR signals are used at twice the frequency, the signal frequency is first of all halved via flipflops before the two channels are linked by the exclusive-OR circuit. This frequency halving is intended to stabilize the duty ratio at exactly 50%.
In order to determine the rotation direction, a development of the invention provides for means to be provided for producing a rotation-direction signal from the measurement signals. The rotation direction can be determined in a manner known per se, for example by linking the measurement signals by means of an edge-controlled flipflop. The rotation-direction signal can be modulated in a manner known per se onto the rotation-frequency signal, for transmission via a two-wire interface. Details relating to this are specified, for example, in Patent Application 198 19 783.7
The two sensor signals required to increase the frequency allow rotation-direction identification, which can be achieved easily, for additional purposes without any major additional complexity.
Further advantageous improvements of the invention are achieved by the measures outlined in the other dependent claims.
Exemplary embodiments of the invention are explained in more detail in the following description and are illustrated in a number of figures in the drawing, in which: